A TV projection lens is generally required to have a large aperture for providing acceptable image brightness and a large field of view for providing a projection TV system which is relatively compact. It is generally preferred that such a lens have an aperture of about f/1.0 and a half-field angle of between about twenty degrees and thirty degrees.
A projection lens is generally not required to have a very high resolution, as a conventional TV image in itself has relatively poor resolution. A resolution standard for a TV projection lens is usually specified as a modulation transfer function (MTF) value. For conventional TV, for example, a modulation of about 0.7 at a spatial frequency of three lines (cycles) per millimeter (3 lines/mm) is usual. For high definition TV, an MTF value of 0.7 at 10 lines/mm is generally specified. By way of comparison, a good quality photographic objective may have an MTF of about 0.7 at about 30 lines/mm.
Although the resolution requirements for a TV projection lens may be relatively undemanding, the combination of large aperture and field angle requirements makes the design of such lenses a difficult task. As many as ten conventional spherical elements may be required to provide an acceptably corrected lens. The cost of such a lens would generally be too high to satisfy a manufacturer's cost goal for a complete projection TV system.
A common design approach which has been adopted for TV projection lenses is to employ a lens element or lens elements having at least one aspheric surface. This reduces the number of lens elements required for the lens, which can reduce cost and provide a more compact lens. Surfaces of the aspheric lens elements in such lenses may depart quite significantly from spherical and are thus not easily fabricated from glass blanks by conventional grinding and polishing methods. For this reason, as well as for cost reasons, they are generally fabricated from plastic. Aspheric plastic elements having complex surface curvatures may be fabricated, for example, by a methods including molding and diamond turning. Several examples of lenses including aspheric elements are described in U.S. Pat. No. 4,900,139.
While plastic lens elements in general may be convenient to produce, they have certain significant shortcomings. The most significant of theses shortcomings is a large variation of optical properties with temperature, compared to glass elements. Additionally, optical properties of plastic elements may also vary significantly with humidity, and plastic elements are relatively soft and easily scratched. Further, only a small range of refractive index values is available for optical plastics, the range of values being between about 1.45 and 1.6. Such values are generally considered low by lens designers. Restriction to such low refractive indices further complicates an already complicated optical design problem.
There is a need for a TV projection lens which is at least as cost effective and compact as lenses employing plastic aspheric elements, but which includes lens elements having only spherical or plane surfaces and preferably only lens elements made from optical glasses.